1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus comprising a droplet ejection head in which a failed ejection detection unit for detecting failed ejections is embedded.
2. Description of the Related Art
Conventionally, one known example of an image forming apparatus is an inkjet recording apparatus (inkjet printer) that has an inkjet head (ink discharge head) with an alignment of multiple nozzles and that forms an image on a recording medium by discharging ink (ink droplets) from the nozzles while moving the inkjet head and the recording medium relative to each other.
Various methods are known in conventional practice as ink ejection methods for such an inkjet recording apparatus. Known examples include a piezoelectric system wherein a vibration plate that constitutes part of a pressure chamber (ink chamber) is deformed by the deformation of a piezoelectric element (piezoelectric ceramics), the capacity of the pressure chamber is changed, ink is led into the pressure chamber from an ink supply channel during this increase in pressure chamber capacity, and the ink in the pressure chamber is ejected as droplets during the decrease in pressure chamber capacity. Further, known examples also include a thermal inkjet system wherein ink is heated to create air bubbles for ejecting the ink by the expansion energy when the air bubbles grow.
In image forming apparatuses that have ink ejection heads such as an inkjet recording apparatus, ink is supplied from an ink storage tank to ink ejection heads via an ink supply channel, and the ink is ejected by the various ejection methods described above, but ejection must be stabilized so that the amount of ink ejected, the rate of ejection, the direction of ejection, and the shape (volume) of the ejected ink and the like are always constant.
However, during printing, the nozzles of the ink ejection heads are always filled with ink so that printing can be immediately performed upon receiving a print command, and the ink in the nozzles is exposed to air; therefore, the ink in the nozzles dries when it is not ejected for a long period of time, the viscosity of the ink increases, rigid ink droplets cannot be ejected, and the nozzles clog, which sometimes results in failed ejections. Also, the refilling of ink is sometimes slowed and unsatisfactory ejections may occur due to the accumulation of the air bubbles that are mixed in the ink supply channel or the like and block the supply of ink, or due to continuing ejections over a long period of time.
These reasons are the causes of failed ejection as described above, and maintenance must be performed on the ejection heads when stable ink ejection is no longer possible. In view of this, various methods have been proposed in conventional practice for determining whether ink has been ejected in a stable manner or if the ejection heads have failed to eject ink.
In one known example, a determination device is disposed for determining the state of displacement of a vibration element that varies the capacity in the ink chamber according to an electrical signal in order to discharge ink from ink droplet discharge ports, and the displacement of the vibration element has a high-frequency vibration component when air is mixed in the ink chambers during the recording operation. Therefore, irregularities in the state of displacement of the vibration element are detected by determining the displacement and identifying the state of failed ejection of the ink (for example, see Japanese Patent Application Publication No. 55-118878).
Another known example is a failed ejection detection method that uses an inverse analysis technique of signal processing, which involves a drive circuit for a piezoelectric element and a circuit for adjusting the vibration waveform during the driving of the piezoelectric element, wherein the high frequency components of the vibration waveform of the piezoelectric element are converted to pulses and extracted, the repeating cycle of the vibration waveform is determined, and the presence or absence of air bubbles in the ink chambers or the state of unfilled ink or the like is determined from the variations in this cycle (for example, see Japanese Patent Application Publication No. 63-141750).
Also, the capacity of the pressure chambers rapidly decreases when the piezoelectric elements are driven by a pulse signal, ink particles are ejected from orifices (ink ejection ports), and the pressure chambers return to their original states when the pulse signal is no longer applied, but the vibration of the ink produced at this time is transmitted to the piezoelectric element and the piezoelectric element creates an electrical signal.
In view of this, in another known example, an inverse analysis technique of signal processing is performed to a higher degree so that the output signal of the piezoelectric element is determined, and at least one of either the extent of the vibration damping time and the peak value range of the output signal is observed to determine which of the pressure chambers or ink chambers contain the air bubbles (for example, see Japanese Patent Application Publication No. 4-29851).
Another known example has a pressure measuring device for determining the internal pressure of the ink supply channel in addition to the piezoelectric element for creating pressure to eject ink, in which a failed ejection detection unit is used to observe the variation in the voltage produced by the deformation of the piezoelectric element due to internal pressure variation in the ink supply channel immediately after ink has been ejected, ejection/failed ejection is determined based on the variations in the voltage produced, and the internal pressure of the ink supply channel as determined by the pressure measuring device is controlled by a pressurizing device on the basis of a failed ejection signal so as to reach a set pressure value, whereby a failed ejection recovery operation is performed (for example, see Japanese Patent Application Publication No. 11-286124). In addition, the example described in Japanese Patent Application Publication No. 11-286124 has a buffer chamber that separates air bubbles in order to prevent the air bubbles from being mixed in the ink.
In another known example, as a developing technique of inverse analysis, an actuator (vibration inducing element) and a reflecting plate are provided to the opposite sides constituting the ink chamber so as to come into direct contact with the ink in the ink chamber, the vibration caused by the actuator is propagated into the ink, and the vibration reflected by the reflecting plate facing the actuator and returned back to the actuator is determined, whereby debris or air bubbles mixed in the ink chamber are detected (for example, see Japanese Patent Application Publication No. 11-309874).
Additionally, as a typical example of an inverse analysis technique, the impedance at an arbitrary frequency of the piezoelectric element in the head is measured, frequency characteristics of the impedance are created, and the frequency characteristics are used to determine whether air bubbles have adhered to the piezoelectric element (for example, see Japanese Patent Application Publication No. 11-334102).
However, the example disclosed in Japanese Patent Application Publication No. 55-118878 has another determination device provided to the vibration element to determine the displacement of the vibration element, but with either a piezoelectric member with a bimorph structure or a push rod-type piezoelectric member, forming more than the minimum intervals necessary to manufacture the vibration element results in portions in which the pressure is lost, which is undesirable for efficiently producing force (pressure) for ejecting ink.
This drawback is particularly pronounced when a high-density head is manufactured. Also, generally the size of the driving element should be taken into greater consideration than the size of the determination element in terms of the function of creating an ink ejection force, but since there is a limit on how small the determination element can be, problems are encountered in that the difference in size between the determination element and the drive element decreases, the ratio of the vibration plate covered by the drive element further decreases as a result, there is a greater loss in force created (pressure) by the drive element, and, depending on the situation, the created force may not be sufficient for ejection.
The examples described in Japanese Patent Application Publication Nos. 63-141750, 4-29851, and 11-334102 are designed to detect failed ejection using a system referred to as an inverse analysis technique that also uses the drive element as the determination element. According to this system, using the drive element itself for ejecting ink as a determination element is advantageous in that no new components are needed, but it has a basic problem in that the drive signal and the determination signal must be separated.
Other problems are encountered in that determination is difficult to perform the moment the drive force for ink ejection is produced because the drive signal suddenly changes and the amplitude becomes too great, resulting in determination results being produced at times other than ejection, which causes the time in which determination is possible to be limited and also limits the information that can be obtained by such a determination; therefore, precise determination is not possible.
The example described in Japanese Patent Application Publication No. 11-309874 can also be considered a developing technique of inverse analysis, designed so that vibration is reflected by another member, it is determined when the vibration has returned to the drive element with a time difference, but the strength of the reflectivity of the reflective surface in contact with the ink must be increased to a certain extent with consideration for the strength of the reflected wave. Also, in order to separate the drive signal and the determination signal, the pulse range (time) of the drive signal and the distance between the drive element and the reflective plate must be increased so as to allow the time difference needed to separate these signals, which creates restrictions on the drive signal waveform and the dimensions of the ink pressure chambers, and these restrictions become more severe when ink is ejected at high speeds. Consequently, problems are encountered in that it is difficult to achieve this configuration, particularly in a high-density head wherein the pressure chambers have been reduced in size.
The example described in Japanese Patent Application Publication No. 11-286124 determines the internal pressure changes in the ink supply channel, which is a problem in that it is unsuitable for detecting pressure irregularities during discharge.
Furthermore, in any of the conventional techniques described above, particularly in a matrix-type head, which is a high density head spanning the page width, no suitable configuration is provided for detecting failed ejections.